1. Field of the Invention
The present invention relates to an optical communication device, in particular to a broadband light source.
2. Description of the Related Art
In constructing wavelength division multiplexing passive optical networks (WDM-PON), which are technology candidates for high-speed fiber-to-the-home (FTTH) applications, a low-price broadband light source are required. Together with a wavelength locked (Fabry Perot laser diode (FP-LD), such a broadband light source plays an important role in concurrently accommodating a plurality of subscribers. In addition, when such a broadband light source is employed in an optical communication system incorporating an erbium doped fiber amplifier (EDFA), it is also needed to measure optical characteristics of the communication components in predetermined signal wavelength bands (e.g. 1530 nm˜1570 nm, 1570 nm˜1610 nm). Existing available broadband light sources mainly employ a white light source incorporating a halogen lamp, an EDFA outputting amplified spontaneous emission (ASE), an edge-emitting light emitting diode (EELED), or a super luminescent diode (SLD). However, white light sources and EELEDs have low output power and are not suitable as a light source for a WDM-PON and SDLs as they output relatively high power. Thus, white light sources and EELEDs are insufficient for use as broadband light sources for a WDM-PON as compared to EDFAs. In fact, EDFAs are commercialized as a broadband light source but are required to have a higher output power over a wide wavelength band when employed as a light source for a WDM-PON. Disadvantageously, however, their construction becomes complicated in order to meet with the above requirements, and thus are not economical in price.
FIG. 1 shows a diagram of a broadband light source according to the prior art. The broadband light source 100 comprises first and second erbium doped optical fibers 140, 145, first and second pump laser diodes 120, 125, first and second wavelength selective couplers (WSCs) 130, 135, a band-pass filter (BPF) 160, and first and second isolators (ISOs) 150, 155. The first wavelength selective coupler 130, the first erbium doped optical fiber 140, the first isolator 150, the band-pass filter 160, the second erbium doped optical fiber 140, the second wavelength selective coupler 135, and the second isolator 155 are connected in series using a first optical waveguide. In addition, the second isolator 155 is connected in parallel to the first erbium doped optical fiber 140 using a second optical waveguide. The second pump laser diode 114 is connected in parallel to the second erbium doped optical fiber 145 using a third optical waveguide.
The first pump laser diode 120 outputs first pump light.
The first wavelength selective coupler 130 is located between a terminal end of the broadband light source 100 and the first erbium doped optical fiber. The first wavelength selective coupler 130 supplies the first pump light to the first erbium doped optical fiber 140.
The first erbium doped optical fiber 140 is located between the first wavelength selective coupler 130 and the first isolator 150. The first erbium doped optical fiber 140 outputs ASE to the front and rear sides as it is pumped by the first pump light. The ASE outputted to the rear side of the first erbium doped optical fiber 140 passes the first wavelength selective coupler 130. Then, the ASE is inputted into the terminal end 102 and disappears. The ASE outputted to the front side of the first erbium doped optical fiber 140 passes the first isolator 150 and the band-pass filter 160, and the ASE is inputted into the second erbium doped optical fiber 145, thus being amplified. Thereafter, the ASE passes the second wavelength selective coupler 135 and the second isolator 155, and the ASE is outputted to the outside through an output end 104 of the broadband light source 100.
The first isolator 150 is located between the first erbium doped optical fiber 140 and the band-pass filter 160. The first isolator 150 passes the ASE inputted from the first erbium doped optical fiber and blocks light progressing in the opposite direction.
The band-pass filter 160 is located between the first isolator 150 and the second erbium doped optical fiber 145. The band-pass filter 160 limits the bandwidth of the ASE passing the first isolator 150 in a wavelength band of 1541 nm˜1559 nm in such a manner that high output power can be obtained in the wavelength band.
The second pump laser diode 125 outputs second pump light.
The second wavelength selective coupler 135 is located between the second erbium doped optical fiber 145 and the second isolator 155. The second wavelength selective coupler 135 supplies the second pump light to the second erbium doped optical fiber 145.
The second erbium doped optical fiber 145 is located between the band-pass filter 160 and the second wavelength selective coupler 135. The second erbium doped optical fiber 145 amplifies and outputs the ASE having passed the band-pass filter 160.
The second isolator 155 is located between the second wavelength selective coupler 135 and the output end 104 of the broadband light source 100. The second isolator 155 passes the ASE having passed the second wavelength selective coupler 135 and blocks light progressing in the opposite direction.
FIG. 2 is a graph showing ASE spectrums in regard to positions of the broadband light source shown in FIG. 1. FIG. 2 shows first spectrum 210 in position A, second spectrum 220 in position B, and third spectrum 230 in position C. The first spectrum 210 has lower output power as compared to the third spectrum 220, and the ASE in position C serves as a seed for the second erbium doped optical fiber 145.
However, such a broadband optical source has a number of limitations, including employing expensive components for generating ASE serving as a seed, thus it is not economical. In addition, the ASE is not effective because the portion of the spectrum beyond a predetermined wavelength band is removed in the band-pass filter 160.